1. Field of the Invention
This invention relates to harmonic drive transmissions, and more particularly to a piezo-electric drive mechanism to rotatably empower that harmonic drive transmission.
2. Prior Art
Harmonic drive transmissions were originally called "strain-wave gearing" and were initially introduced by Musser in U.S. Pat. No. 2,906,143. Such original harmonic drive, strain-wave gearing comprised a rigid circular spline having "N" teeth, a flexspline having fewer than "N" teeth ("N" being a positive integer) and being disposed within the circular spline, and a rotatable wave generator disposed in the flexspline to deform the flexspline into a lobed configuration, such as an oval shape, so as to force the flexspline into an engagement with the circular spline at two points on the major axis of the formed ovaloid.
The wave generator may include an oval cam plate and a bearing snugly mounted on the outer periphery of the cam plate. The outer bearing is matingly inserted into the flexspline so as to deform it to the peripheral contour of the cam plate. An input shaft attached to the cam plate provides rotation thereto, causing the ovaloid configuration of the flexspline to be correspondingly rotated. During such rotation, the circular spline is induced to rotate, relative to the flexspline, an amount proportional to the difference in the number of teeth between the flexspline and the circular spline. When an output shaft is arranged on either the flexspline or the circular spline, that output shaft is rotated very slowly in comparison to its input shaft. Such harmonic drive, strain-wave gearing has been utilized in machinery requiring a high reduction ratio.
There are instances where a small reduction ratio is required, for example, as low as about 50:1. In most cases, in harmonic drive systems, the wave generator is driven by an electric motor. Electric motors however, typically have speed limitations, both on the upper and lower ends. At the upper end of these ratios, the motors and their bearings and those bearings on the wave generator, may wear or burn out rapidly. At the low end of these reduction ratios, the motors will not work well. There is a trade-off between torque and speed in using an electrically driven harmonic drive transmission. High torque will not permit a high-speed output, and a low torque will not permit a low speed output.
It is an object of the present invention to provide a drive means for a harmonic drive transmission, which will satisfy the needs for high torque and high speed and/or low torque and low speed in a manner not found in the prior art.
It is yet a further object of the present invention to provide a harmonic drive transmission which eliminates the drawbacks typically found with wave generators and their bearings.
It is a yet a further object of the present invention to provide a piezo electric drive arrangement for a harmonic drive transmission by a simple and efficient coupling not found in the art.